In this description, an oil well and a gas well are collectively referred to simply as “an oil well.” Therefore, in this description, “a stainless steel for oil well” includes a stainless steel for oil well and a stainless steel for gas well. “A stainless steel pipe for oil well” includes a stainless steel pipe for oil well and a stainless steel pipe for gas well. In this description, “high temperature” refers to temperature of at least 150° C. And in this description, “%” related to elements refers to “percent by mass” unless otherwise specified.
Recent years have seen advances in the development of deep oil wells. Deep oil wells have high-temperature environments. High-temperature environments contain carbon dioxide gas or both carbon dioxide gas and hydrogen sulfide gas. These gases are corrosive gases.
Conventional oil-well environments contain carbon dioxide gas (CO2) and chloride ions (Cl−). For this reason, in conventional oil-well environments, martensitic stainless steels having an excellent carbon dioxide corrosion resistance and containing 13% Cr (hereinafter, referred to as 13% Cr steels) is used.
However, steels for oil well used in the above-described deep oil wells are required to have higher strength and higher corrosion resistance than 13% Cr steels. Dual-phase stainless steels have high Cr contents and have higher strength and higher corrosion resistance than 13% Cr steels. Dual-phase stainless steels are, for example, 22% Cr steels having a Cr content of 22% and 25% Cr steels having a Cr content of 25%. However, dual-phase stainless steels are expensive.
JP2002-4009 (Patent Document 1), JP2005-336595 (Patent Document 2), JP2006-16637 (Patent Document 3), JP2007-332442 (Patent Document 4), JP2006-307287 (Patent Document 5), JP2007-169776 (Patent Document 6) and JP2007-332431 (Patent Document 7) propose other steels which have higher strength and higher corrosion resistance than 13% Cr steels and are different from the above-described dual-phase stainless steels. The stainless steels disclosed in these documents contain 15 to 18% of Cr.
Specifically, Patent Document 1 (JP2002-4009) proposes a high-strength martensitic stainless steel for oil well having yield strength of at least 860 MPa and having carbon dioxide gas corrosion resistance in 150° C. environments. The stainless steel described herein contains Cr: 11.0 to 17.0% and Ni: 2.0 to 7.0% and further has a chemical composition satisfying Cr+No+0.3Si−40C−10N−Ni−0.3Mn≦10. The martensitic stainless steel described herein further has a tempered martensite structure containing at most 10% of retained austenite.
Patent Document 2 (JP2005-336595) proposes a stainless steel pipe having high strength and having carbon dioxide gas corrosion resistance in a 230° C. environment. The chemical composition of the stainless steel pipe described herein contains Cr: 15.5 to 18%, Ni: 1.5 to 5%, and Mo: 1 to 3.5%, satisfies Cr+0.65Ni+0.6Mo+0.55Cu−20C≧19.5, and satisfies Cr+No+0.3Si−43.5C−0.4Mn−Ni−0.3Cu−9N≧11.5. The microstructure of the stainless steel pipe described herein contains 10 to 60% of a ferrite phase and at most 30% of an austenite phase, and the balance is a martensite phase.
Patent Document 3 (JP2006-16637) proposes a stainless steel pipe having high strength and having carbon dioxide gas corrosion resistance in environments of over 170° C. The chemical composition of the stainless steel pipe described herein contains, by mass percent, Cr: 15.5 to 18.5%, Ni: 1.5 to 5%, satisfies Cr+0.65Ni+0.6Mo+0.55Cu−20C≧18.0, and satisfies Cr+Mo+0.3Si−43.5C−0.4 Mn−Ni−0.3Cu−9N≧11.5. The microstructure of the stainless steel pipe described herein may contain an austenite phase or need not contain an austenite phase.
Patent Document 4 (JP2007-332442) proposes a stainless steel pipe having high strength of at least 965 MPa and having carbon dioxide gas corrosion resistance in environments of over 170° C. The chemical composition of the stainless steel pipe described herein contains, by mass percent, Cr: 14.0 to 18.0%, Ni: 5.0 to 8.0%, Mo: 1.5 to 3.5%, and Cu: 0.5 to 3.5%, and satisfies Cr+2Ni+1.1Mo+0.7Cu≦32.5. The microstructure of the stainless steel pipe described herein contains 3 to 15% of an austenite phase, and the balance is a martensite phase.
Patent Document 5 (JP2006-307287), Patent Document 6 (JP2007-169776) and Patent Document 7 (JP2007-332431) disclose stainless steel pipes containing, by mass percent, higher than 15% of Cr. The stainless steel pipes of these documents are expanded after being buried in oil wells. In order to increase expandability, the stainless steels of these documents have high austenite ratios. Specifically, the austenite ratios of the stainless steels of these documents exceed 20%. Or the ratio of austenite to tempered martensite is at least 0.25. The yield strength of the stainless steels of these documents is in many cases at most 750 MPa.
As described above, the stainless steels disclosed in Patent Documents 1 to 7 contain Cr in amounts larger than 13% and contains alloying elements, such as Ni, Mo, and Cu. For this reason, the stainless steels have carbon dioxide gas corrosion resistance in high-temperature environments.